Transistors using nitride semiconductors with electron mobility higher than that of silicon have been researched and developed actively and also put into actual use. For example, gallium nitride (GaN) is a semiconductor having a band gap larger than that of Si. It is therefore possible to produce an element that stably operates in the range of a low temperature to a high temperature as compared to Si and is also capable of high voltage/high current driving. For this reason, a transistor using GaN for a channel is expected to be used as a switching element to be mounted in a consumer electrical appliance or a vehicle.
For example, a so-called two-dimensional electron gas (2DEG) having a high concentration and high mobility is formed in the interface between an AlGaN layer and a GaN layer which are crystal-grown using a c plane as a principal plane. There exists a high electron mobility transistor (HEMT) using the 2DEG as a channel. This transistor can be used as a switching element that exhibits a low ON resistance and a high breakdown voltage. For example, a HEMT using AlGaN/GaN is employed as a transistor for a communication satellite that functions in an environment with a large temperature change from about −150° C. to about 250° C.
To implement such a high-performance transistor that stably operates regardless of a change in the ambient temperature, it is important that, for example, the contact resistance of the source electrode and the drain electrode is low and does not change in accordance with the temperature. However, when a transistor is formed using a nitride semiconductor in which a threading dislocation exists, for example, a nitride semiconductor epitaxially grown on a substrate of a different type such as an SiC substrate, a sapphire substrate, or an Si substrate, in a HEMT using AlGaN/GaN, the contact resistance of the source electrode and the drain electrode changes in accordance with a measurement temperature. For example, it has been reported that in a HEMT using AlGaN/GaN on a Si substrate, when the measurement temperature is changed from room temperature to 200° C., the contact resistance changes to about ⅕ from about 5×10−5 Ωcm2 to about 1×10−5 Ωcm2 (see non-patent literature 1).
A change in the contact resistance depending on the temperature is reported for a HEMT using AlGaN/GaN on a sapphire substrate as well (see non-patent literatures 1 and 2). Such a change in the contact resistance depending on the temperature is a factor for impeding the stable operation of the transistor functioning in an environment with a large temperature change.
The ohmic junction of each of the source electrode and the drain electrode of the HEMT using AlGaN/GaN is generally obtained by depositing metals in the order of Ti, Al, Ni, and Au and heating (sintering) them at a temperature of about 850° C. in a nitrogen atmosphere, as in non-patent literature 1. As the conduction mechanism in the interface between the semiconductor and the metal, three mechanisms, that is, mechanisms by field emission, thermionic field emission, and thermionic emission can mainly be assumed. The conduction mechanisms by thermionic field emission and thermionic emission promote carrier movement on the semiconductor-metal interface along with an increase in the temperature and lower the contact resistance along with the increase in the temperature. On the other hand, in field emission, carrier movement is caused by tunnel conduction, and therefore, the contact resistance does not change depending on the temperature.
In a general ohmic electrode including non-patent literature 1, AlGaN that lacks nitrogen (since a nitrogen vacancy serves as a donor, AlGaN is in an electron-doped state) is generated by the reaction between a metal and a nitride semiconductor (AlGaN), as shown in the band diagram of FIG. 7. The region that lacks nitrogen is defined as a region I. Since the energy barrier of the region I is thin, electrons move between the region and the metal by a tunneling process. That is, electron movement by field emission occurs.
On the other hand, to cause the electrons to move through the energy barrier (region II) between the metal and unreacted AlGaN, thermal energy is needed to move across the energy barrier. Hence, if the temperature rises, the electrons readily move across the energy barrier of the region II. For this reason, the contact resistance lowers along with the increase in the temperature. This is the very conduction mechanism by thermionic field emission. In non-patent literature 1, it is reported that in AlGaN/GaN on an Si substrate having a threading dislocation density on the order of 109 cm−2, since the conduction mechanism by thermionic field emission via the region II is dominant, temperature dependence of the contact resistance is generated.
Additionally, as described in non-patent literature 2, in AlGaN/GaN on a sapphire substrate generally having a threading dislocation density on the order of 109 cm−2, a structure in which part of a metal contacts GaN via a threading dislocation is generated by a sintering process. The interface between GaN and the metal in contact with GaN via the threading dislocation forms a Schottky junction. Temperature dependence of the contact resistance is considered to be generated because carrier conduction in the Schottky junction is dominated by thermionic field emission.